This invention relates to apparatus for recording various information such as characters, figures, images and audio sound on a disc-like recording medium such as an optical disc.
Magnetic recording media such as magnetic tapes and magnetic discs are widely used in the auxiliary memories for computers. In recent years, auxiliary memory using optical magnetic media, particularly optical discs has become the object of public attention because the optical magnetic media can record information at a much greater density than the magnetic recording media.
In the fields of information service and so on, there is a demand for an image filing apparatus which can conveniently make and store data from still pictures or moving pictures. Particularly, there is a strong demand for an image filing apparatus capable of recording information for characters and figures and image information for still pictures or moving pictures on one disc and capable of processing such information. Also, an optical disc has the features of large capacity and random access and is thus suited as a recording medium for such information.
The information to be processed in the above new field can be roughly classified into information for characters, figures, audio sound and still pictures of which the capacity is relatively small even in codes, and image information of moving pictures or the like, of which the capacity is large. In the conventional code information recording apparatus using optical discs, units of constant amounts of data are digitally recorded on a large number of sectors each of an equal recording capacity each into which track of the disc is divided for random access and information on each sector is added with an error correction code for improving the reliability of the information. In this case, since the signal has low frequency components close to DC in its frequency spectrum, a DC coupling laser recording circuit is used. On the other hand, when image information such as moving pictures is recorded, it is necessary to access each unit of field or frame, and thus each track of the disc includes one or two sectors for recording analog signals unlike those recording code information. In this case, since, before recording, the analog signal is generally frequency-modulated into a signal with no DC component, an AC-coupled (for cutting the components close to DC) laser recording circuit is used. Thus, the conventional optical disc recording apparatus processes coded information such as characters, digitally figures and image information, such as moving pictures, as an analog signal, respectively and records them in different formats, and through different laser recording circuits on separate discs, because these different forms of information have their own properties.
Therefore, when the code information and image information are simultaneously edited and processed, separate optical disc recording apparatus must be used at the same time. Moreover, there is the problem that different management of information and control of the recorders is necessary between different recording media. To cope with this problem, apparatus for recording different information on one disc of an existing medium has been considered. For example, on the laser disc moving pictures and audio sounds are recorded as analog signals, and another audio sound as a digital signal in a frequency divided manner. As to the compact disc (CD), audio sounds and still pictures are recorded as digital signals on a single optical disc. These discs, however, are always used as recorded discs to be repeatedly reproduced, and cannot be used for recording the information individually and independently. For example, it is impossible to record still pictures and later record audio sound. In Japanese Patent laid-open publication No. 64-19556, an optical disc recording apparatus for recording analog and digital signals together is disclosed, but the laser recording circuit is not described which considers the difference between the frequency bands of the analog and digital signals to be recorded, particularly the processing of low-frequency components having a large effect on the servo characteristics.
In addition, from the standpoint of information quality it is desired to record all information in digital signals because digital signals are not deteriorated by processing such as editing. However, aside from still pictures, information of moving pictures are difficult to digitize because even NTSC moving pictures, when digitized, must be recorded at a rate as high as 60 mega-bits/sec, which is another difficulty. In order to record information of characters and figures and the moving pictures on one optical disc through a single recording circuit, it is necessary to use a laser recording circuit through which these types of information can be recorded at a higher rate, or 60 mega-bits/sec. To do this, it is necessary to use a special high-performance pickup having a high-output power semiconductor laser and a wide-band drive circuit for this laser. On the other hand, since with character and figure it is never necessary to record at such a high rate, reducing the amount of information for moving pictures by the use of an image compression technique and recording at a low rate can be considered. However, the technique of image compression with less deterioration of picture quality is still being studied and developed. In either case, encoding all kinds of information including moving pictures, and recording those in a uniform manner has presented many technical problems.